The capability of delivering more than 1 mW of average power at frequencies above about 2 THz combined with very good intrinsic frequency definition make QCLs an appealing solid-state solution as compact THz sources. However, several challenges to the implementation of QCLs as practical THz sources remain. Among these challenges are to couple the QCL power efficiently to other devices (possibly on the same chip) or into free space. To couple the power efficiently to other devices on the same chip, low-loss waveguides are desired, whereas to efficiently propagate the QCL power off the chip requires shaping the non-optimal output beam patterns observed from QCLs into a more useful and predictable beam shape which can be propagated and recoupled to other devices.
There are essentially two types of waveguides of THz QCLs, plasmon and metal-metal. For both, the beam patterns found in the literature show far-field patterns with complex phase structures that are very detrimental for coupling a laser to coherent mixers. See A. J. L. Adam et al., “Beam patterns of terahertz quantum cascade lasers with subwavelength cavity dimensions,” Appl. Phys. Lett. 88(15), 151105 (2006); and E. E. Orlova et al., “Antenna Model for Wire Lasers,” Phys. Rev. Lett. 96, 173904 (2006). Even when the beam is nominally Gaussian, poor coupling to external coherent detectors is observed, possibly due to a large number of emitted modes with different phases. See H. Richter et al., “Terahertz heterodyne receiver with quantum cascade laser and hot electron bolometer mixer in a pulse tube cooler,” Appl. Phys. Lett. 93, 141108 (2008). Groups have tried various approaches to improve the beam quality over the last couple of years. These include soldering a triangular piece of metal on to the end of a QCL, sandwiching the laser between two pieces of silicon to make a capacitive waveguide and horn, placing a silicon lens in contact with the facet, butt-coupling a QCL into a hollow pyrex tube and, more recently, distributed feedback third order Bragg gratings and etched semiconductor horns. See S. Barbieri et al., “Integrated Horn Antenna for Microstrip Waveguide THz Quantum Cascade Lasers,” CLEO Abstracts, CWP4 (2007); M. Amanti et al., “Horn antennas for terahertz quantum cascade lasers,” Elec. Lett. 43, 573 (2007); A. W. M. Lee et al., “High-power and high temperature THz quantum-cascade lasers based on lens-coupled metal-metal waveguides,” Opt. Lett. 32(19), 2840 (2007); A. A. Danylov et al., “Transformation of the multimode terahertz quantum cascade laser beam into a Gaussian, using a hollow dielectric waveguide,” Appl. Opt. 46(22), 5051 (2007); M. I. Amanti et al., “Low-divergence single-mode terahertz quantum cascade laser,” Nat. Phot. 3, 586 (2009); and J. Lloyd-Hughes, “Coupling terahertz radiation between sub-wavelength metal-metal waveguides and free space using monolithically integrated horn antennae,” Opt. Exp. 17(20), 18387 (2009). While each of these approaches improved the beam quality, the first four required individual microassembly by hand, which are not inherently mass-manufacturable in an economic and scalable parallel fabrication process, and the last still couples to multiple modes.
While some of these previous approaches (in particular the 3rd order DFB) are beginning to provide reasonable coupling to free space, none of these are good for guiding THz radiation intra-chip to couple to other THz devices. Such THz photonic integrated circuits (ICs) would help close the THz technology gap between microwave electronics and infrared photonics, which currently have much greater functionality because they already integrate multiple devices on the same chip. Traditional waveguides fabricated on semiconductor platforms, such as dielectric guides in the infrared or co-planar waveguides in the microwave regions, suffer high absorption and radiative losses in the THz. The former leads to very short propagation lengths, while the latter leads to unwanted radiation modes and/or crosstalk in integrated devices. For this reason the waveguide of choice at these frequencies are metallic, hollow, rectangular waveguides. For the most part, these waveguides are fabricated in two halves which are sandwiched together to form the enclosed space. As frequencies exceed 1 THz traditional machining becomes much more difficult and prohibitively expensive for broad use, thus micromachining techniques are being developed. See, e.g., P. L. Kirby, “Characterization of Micromachined Silicon Rectangular Waveguides at 400 GHz,” IEEE Micro. Wire. Comp. Lett. 16(3), 366 (2006); and V. Desmaris, “All-metal micromachining for the fabrication of sub-millimeter and THz waveguide components and circuits,” J. Micromech. Microeng. 18, 095004 (2008). These split blocks require careful insertion of and subsequent connection by hand to active devices. Only a couple efforts have tried to fabricate monolithic waveguides on a chip with or without integrated devices. See J. W. Bowen, “Micromachined waveguide antennas for 1.6 THz,” Elec. Lett. 42(15), (2006); and H. Kazimi, “Active micromachined integrated terahertz circuits,” Int. J. Inf. Millimeter Waves 20(5), 967 (1999).
However, a need remains for a laser source monolithically integrated with a hollow waveguide on the same chip. This integration would allow coupling to mature rectangular waveguide components and circuits, guiding the emission of QCL radiation around on a chip and controlling the emission beam pattern and number of radiated modes by controlling the shape of the waveguide and horn antennas coupled to the waveguides. In addition, by altering the geometry of the interface between the laser and rectangular waveguide, the coupling of the QCL to the outside world could be controlled.